1. Field of the Invention
This invention relates to the field of digital scanners capable of generating digital signals from scanned text or graphics.
2. Description of the Related Art
Digital scanners typically are used to convert text or graphics on a physical medium, such as paper, to digital signals. Digital scanners include moving-document and fixed-document types. In moving-document types, the physical medium is moved over a receiving portion of the scanner. Fixed document scanners are compactly designed so that they can be moved across a surface of the information-bearing physical medium for reading the information.
Referring now to FIG. 1, a conventional digital scanner 10 typically includes control processor 15, analog-to-digital (A/D) converter 19 and buffer 18. Scanner 10 is coupled to computer 20 over bi-directional signal bus 16. Conventional computers, such as computer 20, typically include memory 22 and processor 24.
A functional block diagram of scanner 10 is shown in FIG. 2. Scanner 10 further includes a light source 11 for illuminating the physical medium, such as document 12. Document 12 typically is placed on a platen of scanner 10 (not shown in FIG. 2). Lens 13 forms an image of an object on document 12 onto detector 14. Detector 14 typically includes an array of elements, each of which converts incident light into an electrical charge or analog signal. Detector 14 can be, for example, a photodiode array, a charge-coupled device array or a charge-coupled photodiode device array.
As noted above, a scanner can be either of the moving-document or fixed-document type. A moving-document scanner typically uses mechanical structure to move the document 12 past detector 14, which remains stationary. A fixed-document scanner captures images by moving detector 14 relative to document 12, or else the entire object on document 12 may be captured at once on a fovea of detector 14.
The analog signal generated by detector 14 which forms the image of an object on document 12 is transmitted to A/D converter 19 for conversion into a digital image signal. The digital signal from A/D converter 19 is coupled to control processor 15. Control processor 15 typically includes hardware and software designed to manipulate the digital image signal from A/D converter 19, for example to improve image quality. For example, control processor 15 typically performs image enhancement and noise removal on the digital image signal. In the case of a text image, control processor 15 typically identifies individual characters in the image, in which case the scanner functions as an optical character recognition (OCR) system. One manner of character recognition involves comparing individual characters to a stored template of characters to be recognized.
Control processor 15 typically provides error correcting, for example, to correct character recognition errors or the like. For example, control processor 15 might include a spell-checking program for verification of word spelling to correct any erroneous text recognition.
Embodiments of control processor 15 also provide signal compression, especially where the digital image signal representing the object on document 12 comprises a large number of bits, such as when the object on document 12 includes color graphics or the like. Control processor 15 also can generate color transforms as necessary, such as by transforming an image in the RGB color space to the YUV or YIQ color space, and may subsample as necessary as a means for compressing the transformed image.
The high latency and low bandwidth of bus 16 limits the rate of transfer of digital image signals from scanner 10 to computer 20 to such an extent that interim storage of the digital image signals is required. On-board buffer 18 is provided for the interim storage. Digital image signals from buffer 18 are transmitted to computer 20 over bus 16 at a rate allowed by bus 16. When scanning multiple documents in relatively rapid succession, digital image signals from a plurality of images will tend to accumulate in buffer 18, because the bandwidth limitations of bus 16 prevent real-time transfer of the signals. A danger exists that buffer 18 will fill to capacity, resulting in an inability to add additional signals and a loss of some digital image signals.
The high latency and low bandwidth of bus 16 therefore limits real-time use and display of the digital image signals, since the digital image signals can not be transmitted from scanner 10 to computer 20 at a sufficiently high rate. In addition, bus 16 subjects the digital image signals to a risk of loss in the event of saturation of buffer 18. Moreover, the need for including buffer 18 in the hardware of scanner 10 adds to the cost of manufacturing scanner 10.
Digital image signals from buffer 18 can be used by an application running on computer 20, such as a spread sheet program, word processing program or the like, in cases where the image being scanned is a text image, or otherwise decompressed and displayed on a video monitor associated with computer 20, where the image is graphical. Of course, the digital image signals also may be stored in memory 22 on computer 20 for later use.